Method for Producting a Thermoelectric Material

ABSTRACT

A thermoelectric material to exploit a unidirectional thermal gradient for the production of electrical power, comprising a body fabricated from milled silicon alloyed with a dopant and sintered at a temperature below the melting point of silicon.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. ProvisionalPatent Application Ser. No. 61/812,095, filed Apr. 15, 2013, thecontents of which is herein incorporated in its entirety by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of Invention

The present general inventive concept relates to the preparation and useof silicon-based materials with thermoelectric properties.

2. Description of the Related Art

The biggest sources of waste heat include electrical generators, steeland glass production, the use of combustible fuels for transportation,oil refineries, as well as the loss and cost of disposal of heat interms of fans and heat sinks for electronic semiconductors and powerdevices, air conditioning, and other low temperature applications. Theelectrical efficiency of power plants for instance, the ratio betweenenergy used in the process and the energy extracted is about 30%.

Physics offers a path for recovery of most all waste heat, from lowtemperature to high temperature, by exploiting the use of semiconductivematerials exhibiting the Seebeck effect where electrons are bothcarriers of electricity and heat. This class of material is calledthermoelectrics. Both n-type and p-type thermoelectrics must be used intandem to produce power from waste heat or to (reversibly) use power toinstead cool or heat through solid-state means (i.e., no moving partsexcept electrons). When a uniaxial temperature gradient is presentwithin a TE structure, a colder and a warmer, a diffusion of electronsis created from the hot side toward the cold side, forming two poles ofan electrical field. The voltage measured therein called Seebeckvoltage. The Seebeck coefficient (S) has units of voltage per unittemperature and is a material property.

There exists a need to find a material for use as a thermoelectricsemiconductor capable of being molded or fabricated into shapes toexploit a unidirectional thermal gradient for the production ofelectrical power.

Most contemporary thermoelectric materials and their compounds sufferfrom a variety of ills, including high cost, difficulty in processing,poor performance, use of precious or potentially carcinogenic elements,limited freedom of the engineer to manipulate properties such aselectrical resistivity and thermal conductivity, thermal shocksusceptibility, shape, size, indexing these properties from the hot sideto the cold side and many others. For this reason, progress has beenslow and unaccretive with regard to ZT and operating temperature andsize for a very long time. None of those in the current road map use myapproach of nano silicon porous structures, though all advances as citedabove use nanostructuring to realize the benefits of the quantum sizeeffects on thermal conductivity.

Materials such as bismuth telluride (Bi₂Te₃) and bismuth selenide(Bi₂Se₃) comprise some of the best performing low-temperaturetemperature thermoelectrics with a temperature-independent ZT between0.8 and 1.0. These materials are used for solid-state cooling in smallconsumer refrigerators and for solid-state heating in some styles ofmosquito traps. Nanostructuring these materials to produce a layeredsuperlattice structure of alternating Bi₂Te₃ and Bi₂Se₃ layers producesa device within which there is good electrical conductivity butperpendicular to which thermal conductivity is poor. The result is anenhanced ZT (approximately 2.4 at room temperature for p-type), but thishigh value has not been independently substantiated. However, theavailability of elements such as tellurium and selenium may soon be injeopardy as reports indicate both are past their production peaks.Therefore it is desirable to identify thermoelectric materials havinglong-term abundance (e.g., silicon).

Skutterudite thermoelectrics are of contemporary interest because oftheir medium to high-temperature use capability. These structures are ofthe form (Co,Ni,Fe)(P,Sb,As)₃. Unfilled, these materials contain voidsinto which low-coordination ions (usually rare earth elements) can beinserted to alter thermal conductivity by producing sources for latticephonon scattering and decrease thermal conductivity due to the latticewithout reducing electrical conductivity. However, the processing ofmaking dense skutterudite thermoelectrics is not trivial. It involvespowder preparation methods and billet formation methods that are notparticularly well-suited for mass production or net shape processing.Al₂O₃ while being inexpensive and a low dielectric, has a high CTE and alow thermal conductivity.

BeO has a high thermal conductivity, low dielectric, but is toxic andexpensive and has a high CTE.

AlN has a low dielectric, and high thermal conductivity and a low CTE,but is expensive.

Glass, including fused silica, is inexpensive, has a low dielectric, alow CTE and a very low thermal conductivity.

To date, silicon has not been widely used, possibly because of thedanger of milling silicon to sizes in the micron to submicron range.While many of the factors for silicon have been proven, for instanceability to engineer low thermal conductivity, dope to increaseelectrical conductivity, relative insensitivity of silicon's SeebeckCoefficient to doping and electrical conductivity, high operatingtemperature, the approaches taken so far are mostly expensive, sizelimiting and process limiting approaches such as nano wires and MEMS.

BRIEF SUMMARY OF THE INVENTION

The present invention, in some of its several embodiments, comprisesthermoelectric materials and methods and processes for making the same.In some embodiments, the thermoelectric materials include silicon milledaccording to a process taught by U.S. Pat. No. 6,638,491.

Some example embodiments of the present invention comprise an alloy ofsilicon milled with some other material (or “dopant”), wherein thedopant is added to the melt during a late or final step of the siliconrefinement process. In many cases, the dopant added to the melt andalloyed with the silicon is selected so as to optimize the desiredelectrical properties (e.g., n-type or p-type semiconductor). Inparticular, for n-type semiconductors, arsenic is attractive and usefuldopant due to the large size of the lattice with arsenic (compared to,e.g., phosphorous) and the comparatively low mobility through thermaldiffusion.

The silicon and dopant are milled and formed into the desired shape orshapes. These “green” production preforms can include establishedprocesses including tape casting, iso-pressing, dry pressing, slipcasting, injection molding, and the like, which means one can make veryinexpensive and yet complex net shapes. Such processes allow for thefabrication of shapes that fully exploit a unidirectional thermalgradient.

In many cases, the formation of planer or complex shapes includes atwo-step process in which one side is fabricated with silicon doped witha first dopant so as to be n-type and the other side is fabricated withsilicon doped with a second dopant so as to be a p-type semiconductor.This combined with the tailored electrical and thermal conductivityproperties allows for the production of a Seebeck device of optimalefficiency and performance.

This thermoelectric material is then “sintered” into a polycrystallineform and shape with controlled porosity and density and high intrinsicmechanical strength. The sintering process is a solid state diffusionalprocess wherein the grains contact each other and bond at a temperaturebelow the melting temperature of the silicon.

Relatively pure bulk silicon preforms have been produced by dry pressingand sintering in argon (or some other inert gas or atmosphere) at atemperature less than the melting point of silicon (approx. 1414° C.).Such processes result in thermoelectric structures with controlledelectrical resistivity and dramatically reduced thermal conductivitycompared to competing materials. Pure, single-crystal silicon hasthermal conductivity of 149 watts per meter-Kelvin (W/m·K), yetprocesses according to embodiments of the present general inventiveconcept have made bulk sintered polycrystalline silicon thermoelectrics(SinPolySiTEs) with controlled electrical resistivity and thermalconductivity down to 10 W/m·K. Further decreases in K are achievablewith more careful control of porosity, control of grain size and grainsize distribution of the milled silicon powder, and the introduction ofan appropriate secondary phase.

In some example embodiments of the present general inventive concept, aprocess for fabricating a thermoelectric material includes admixing aquantity of silicon metal particulates with a liquid having the abilityto limit oxidation of the silicon metal particulates, said step ofadmixing maintained for a time sufficient for wetting the first quantityof silicon metal particulates in the liquid prior to attrition todevelop an oxidant free mixture of particulates and liquid, introducingsaid oxidant-free mixture of particulates and liquid into an attritionmill, said step of introducing proceeding in the absence of oxidants,subjecting said silicon metal particulates of said mixture to attritionin the attrition mill for a time sufficient to reduce at least a portionof said silicon metal particulates to a preselected average particlesize, said liquid limiting oxidation of said silicon metal particulatesduring this time, to produce a second quantity of reduced particle sizesilicon metal particulates being essentially oxidant free, withdrawingfrom said attrition mill at least a portion of said second quantity ofreduced particle size silicon metal particulates, along with a portionof said liquid. providing an initial feedstock of silicon metalparticulates, mixing the milled silicon metal particulates with a dopantto form a thermoelectric material, and sintering the milled siliconmetal particulates and dopant at a temperature below the melting pointof silicon.

In some embodiments, the dopant is a material selected in order to makethe thermoelectric material an n-type semiconductor.

In some embodiments, the dopant is a material selected in order to makethe thermoelectric material a p-type semiconductor.

In some embodiments, the dopant is arsenic.

In some embodiments, the thermoelectric material includes two sides,wherein a first side is an n-type semiconductor and a second side is ap-type semiconductor.

In some embodiments, the preselected average particle size is less than1,000 nanometers.

In some embodiments, the preselected average particle size is less than600 nanometers.

In some embodiments, the preselected average particle size is less than300 nanometers.

In some example embodiments of the present general inventive concept, amethod for fabricating a thermoelectric material includes providing aninitial feedstock of silicon metal particulates, providing an extractingliquid to extract oxidants from the silicon metal particulates,combining the silicon metal particulates and the extracting liquid intoa mixture and milling said mixture, withdrawing at least a portion ofthe milled mixture, within the withdrawn portion of the milled mixture,separating milled silicon metal particulates from the extracting liquid,and mixing the milled silicon metal particulates with a dopant to form athermoelectric material.

In some embodiments, the method includes the further step, followingmixing the milled silicon metal particulates with a dopant, of sinteringthe milled silicon metal particulates and dopant at a temperature belowthe melting point of silicon.

In some embodiments, the dopant is a material selected in order to makethe thermoelectric material an n-type semiconductor.

In some embodiments, the dopant is a material selected in order to makethe thermoelectric material a p-type semiconductor.

In some embodiments, the dopant is arsenic.

In some embodiments, the thermoelectric material includes two sides,wherein a first side is an n-type semiconductor and a second side is ap-type semiconductor.

In some example embodiments of the present general inventive concept, athermoelectric material to exploit a unidirectional thermal gradient forthe production of electrical power includes a body fabricated frommilled silicon alloyed with a dopant and sintered at a temperature belowthe melting point of silicon.

In some embodiments, the dopant is a material selected in order to makethe thermoelectric material an n-type semiconductor.

In some embodiments, the dopant is a material selected in order to makethe thermoelectric material a p-type semiconductor.

In some embodiments, the body includes two sides, a first side being ann-type semiconductor and a second side being a p-type semiconductor.

In some embodiments, the milled silicon includes particles with anaverage particle size is less than 1,000 nanometers.

In some embodiments, the milled silicon includes particles with an theaverage particle size is less than 600 nanometers.

In some embodiments, the milled silicon includes particles with an theaverage particle size is less than 300 nanometers.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The above-mentioned features and other aspects of the invention willbecome more clearly understood from the following detailed descriptionof the invention read together with the drawings in which:

FIG. 1 is a flow diagram of an example embodiment of a process forfabricating a thermoelectric material.

DETAILED DESCRIPTION OF THE INVENTION

Disclosed herein are silicon-based thermoelectric materials and methodsfor making the same. In some embodiments, the thermoelectric materialsinclude silicon milled according to a process taught by U.S. Pat. No.6,638,491.

Some example embodiments of the present invention comprise an alloy ofsilicon milled with some other material (or “dopant”), wherein thedopant is added to the melt during a late or final step of the siliconrefinement process. In many cases, the dopant added to the melt andalloyed with the silicon is selected so as to optimize the desiredelectrical properties (e.g., n-type or p-type semiconductor). Inparticular, for n-type semiconductors, arsenic is attractive and usefuldopant due to the large size of the lattice with arsenic (compared to,e.g., phosphorous) and the comparatively low mobility through thermaldiffusion.

The silicon and dopant are milled and formed into the desired shape orshapes. These “green” production preforms can include establishedprocesses including tape casting, iso-pressing, dry pressing, slipcasting, injection molding, and the like, which means one can make veryinexpensive and yet complex net shapes. Such processes allow for thefabrication of shapes that fully exploit a unidirectional thermalgradient.

In many cases, the formation of planer or complex shapes includes a twostep process in which one side is fabricated with silicon doped with afirst dopant so as to be n-type and the other side is fabricated withsilicon doped with a second dopant so as to be a p-type semiconductor.This combined with the tailored electrical and thermal conductivityproperties allows for the production of a Seebeck device of optimalefficiency and performance.

This thermoelectric material is then “sintered” into a polycrystallineform and shape with controlled porosity and density and high intrinsicmechanical strength. The sintering process is a solid state diffusionalprocess wherein the grains contact each other and bond at a temperaturebelow the melting temperature of the silicon. The sintering process,generally carried out in a vacuum or ear vacuum, and at a temperaturebelow the melting point of the silicon, does not significantly increasethe density of the thermoelectric material (which would increase thethermal conductivity of the thermoelectric material).

Relatively pure bulk silicon preforms have been produced by dry pressingand sintering in argon (or some other inert gas or atmosphere) at atemperature less than the melting point of silicon (approx. 1414° C.).Such processes result in thermoelectric structures with controlledelectrical resistivity and dramatically reduced thermal conductivitycompared to competing materials. Pure, single-crystal silicon hasthermal conductivity of 149 watts per meter-Kelvin (W/m·K), yetprocesses according to embodiments of the present general inventiveconcept have made bulk sintered polycrystalline silicon thermoelectrics(SinPolySiTEs) with controlled electrical resistivity and thermalconductivity down to 10 W/m·K. Further decreases in K are achievablewith more careful control of porosity, control of grain size and grainsize distribution of the milled silicon powder, and the introduction ofan appropriate secondary phase.

The use of an inert atmosphere for the sintering process (in manyexample embodiments an argon atmosphere) is useful for preventingoxidation of the milled silicon and for preventing other undesirableside effects. It has been found that very low levels of oxygen willoxidize the surfaces of small pieces of silicon. Also, atmosphericcarbon monoxide will both oxidize the surface of the silicon and formsilicon carbide. This can form a “skin” which will retain its shape tovery high temperatures and could preclude the formation of these manybonds particle to particle where the electrical conductivity ismaintained very high and oxides or silicon carbide do not form and limitthis sintering or interfere with electrical conductivity.

Given the design of the tools and the materials of construction verycommon to these furnaces and tools, often or nearly always includingcarbon or graphite, there are normally several sources of oxygen, beingcarbon monoxide or carbon dioxide, silicon monoxide or dioxide, oxygenand the like. In the case of the silicon stealing oxygen from the extantcarbon oxide gas, this is doubly lethal to our process as it will formboth silicon oxide and silicon carbide on the surface of the particles.One must control the oxygen and partial pressure of oxygen andconstituents such as carbon monoxide to very low levels, such low levelsthat the surface area of silicon and graphite and carbon presents manytimes more atoms than is present in the atmosphere.

The level of oxygen available to react with the silicon must be reduced,in the preferred embodiment to significantly less than 10¹⁷ molecules ofoxygen per liter, preferably 10¹⁴ molecules per liter or less, or to putit another way, the number of atoms of silicon and graphite in thesystem should vastly outnumber the number of atoms of oxygen. A vacuumhas been shown to work in some embodiments of the present generalinventive concept. In cases using a vacuum, a 0.2 micron vacuum reducesthe molecules of oxygen in a liter of space by 99.999978%, which meansthe oxygen in a liter of space is reduced from 3.18×10²¹ to 8.36×10¹⁴.At the same time, in a typical environment the number of atoms ofsilicon and graphite available for reaction might be 10²¹ or much more,meaning that very little of the silicon is oxidized to make SiO orreacted with carbon to make SiC.

In one embodiment, the sintering atmosphere is created by using a vacuumfurnace, first purged with argon, then at a low temperature beforeoxygen can react with carbon, evacuated to form a vacuum to a typicallevel of pressure equal to 0.2 microns of mercury. In such a case theoxygen level is in the range of 8.36×10¹⁴ and the oxygen level is so lowit precludes the formation of the oxide of silicon or the carbide ofsilicon in the areas where the silicon particles are to be bondedtogether.

In some example embodiments of the present general inventive concept,the porosity of the structures thus made is reticulated, meaning that awide variety of materials can be “infiltrated” or filled into the bodyof the silicon forms (or between the silicon grains) to further modifyand control thermal conductivity, electrical conductivity, and Seebeckcoefficient (parameters that, in concert, dictate the TE effect). Thisallows for two means of control of the TE effect via (1) thepolycrystalline Si grains themselves and (2) any introduced thesecondary, grain boundary phase. Impregnation can also be used to givethe preform strength for very precise machining operations where billetscan, for instance, be preformed by iso-pressing, and then complex shapescan be machined by industrial machining operations.

To decrease thermal conductivity of the thermoelectric material whilenot interfering with the electrical function, the reticulated spaces andsurfaces are coated or filled with a glass coating through the use ofcolloidal silica or ethyl silicate as a precursor. The benefits of thisinclude giving the silicon protection from oxidation and decreasing thethermal conductivity of the structure, the glass having a theoreticalthermal conductivity of about 1.4 W/m·K.

Some example embodiments of processes and methods according to thepresent general inventive concept start with raw materials doped at highlevels of purity and precision to provide n-type and p-typesemiconductor performance with very low electrical resistivity, in therange of 0.002 ohms/cm² or lower, providing electrical conductivity inthe range of tens of thousands of Siemens/meter. Silicon and dopants aremilled in ethanol using the Carberry '462 patent, reducing the siliconto a particle size less than one micron, preferably in the range of afew hundred nanometers, while keeping the silicon pure and free ofoxidation. The resulting porous pellets or other forms of milled siliconand dopants thus fabricated are then pressed and sintered in a furnacewhere the availability of oxygen and contributors of oxygen are kept atexceptionally low levels, perhaps by a vacuum in the range of 0.2microns of mercury pressure, perhaps in an atmosphere of argon or otherinert gas.

Dopants minimize the electrical resistivity in both n-type and p-typesemiconductors, conversely maximizing electrical conductivity andthereby optimizing the ZT function in the formula where the nominator isthe Seebeck coefficient squared times the electrical conductivitydivided by the thermal conductivity, all of this factored by theabsolute temperature. In some embodiment, the dopant is selected todeliver an electrical resistivity of no more than 2 m-ohms. Theabove-discussed doping may occur, for example, either by providing aninitial feedstock comprising high-purity silicon and an appropriatedopant, or by adding dopant to silicon during the milling process. Incertain embodiments, the dopant may be selected from the groupconsisting of arsenic, phosphorous, boron, and gallium. However, othersuitable dopants may be used without departing from the spirit and scopeof the present general inventive concept.

In some example embodiments of the present general inventive concept,silicon is milled according to a method disclosed in U.S. Pat. No.6,638,491, issued to Carberry. In such a case the use of this technologyis helpful in that it provides for a safe cost effective way to millsilicon.

Turning to FIG. 1, in some example embodiments of the present generalinventive concept, a process for fabricating a thermoelectric materialincludes a number of steps. A quantity of silicon metal particulates areadmixed 100 with a liquid having the ability to limit oxidation of thesilicon metal particulates, said step of admixing maintained for a timesufficient for wetting the first quantity of silicon metal particulatesin the liquid prior to attrition to develop an oxidant free mixture ofparticulates and liquid. The oxidant-free mixture of particulates andliquid is introduced 200 into an attrition mill, said step ofintroducing proceeding in the absence of oxidants, subjecting saidsilicon metal particulates of said mixture to attrition in the attritionmill 250 for a time sufficient to reduce at least a portion of saidsilicon metal particulates to a preselected average particle size, saidliquid limiting oxidation of said silicon metal particulates during thistime, producing a second quantity of reduced particle size silicon metalparticulates being essentially oxidant free. At least a portion of saidsecond quantity of reduced particle size silicon metal particulates,along with a portion of said liquid, is withdrawn 300 from saidattrition mill. The milled silicon metal particulates are mixed (orallyed) with a dopant 400 to form a thermoelectric material. Then, themilled silicon metal particulates and dopant are sintered 500 at atemperature below the melting point of silicon.

While the present invention has been illustrated by description ofseveral embodiments and while the illustrative embodiments have beendescribed in considerable detail, it is not the intention of theapplicant to restrict or in any way limit the scope of the appendedclaims to such detail. Additional advantages and modifications willreadily appear to those skilled in the art. The invention in its broaderaspects is therefore not limited to the specific details, representativeapparatus and methods, and illustrative examples shown and described.Accordingly, departures may be made from such details without departingfrom the spirit or scope of applicant's general inventive concept.

What is claimed is:
 1. A process for fabricating a thermoelectricmaterial, comprising: admixing a quantity of silicon metal particulateswith a liquid having the ability to limit oxidation of the silicon metalparticulates, said step of admixing maintained for a time sufficient forwetting the first quantity of silicon metal particulates in the liquidprior to attrition to develop an oxidant free mixture of particulatesand liquid; introducing said oxidant-free mixture of particulates andliquid into an attrition mill, said step of introducing proceeding inthe absence of oxidants; subjecting said silicon metal particulates ofsaid mixture to attrition in the attrition mill for a time sufficient toreduce at least a portion of said silicon metal particulates to apreselected average particle size, said liquid limiting oxidation ofsaid silicon metal particulates during said time, to produce a secondquantity of reduced particle size silicon metal particulates beingessentially oxidant free; withdrawing from said attrition mill at leasta portion of said second quantity of reduced particle size silicon metalparticulates, along with a portion of said liquid, mixing the siliconmetal particulates with a dopant to form a thermoelectric material; andsintering the milled silicon metal particulates and dopant at atemperature below the melting point of silicon.
 2. The process of claim1 wherein the dopant is a material selected in order to make thethermoelectric material an n-type semiconductor.
 3. The process of claim1 wherein the dopant is a material selected in order to make thethermoelectric material a p-type semiconductor.
 4. The process of claim1 wherein the dopant is arsenic.
 5. The process of claim 1 wherein thethermoelectric material includes two sides, wherein a first side is ann-type semiconductor and a second side is a p-type semiconductor.
 6. Theprocess of claim 1 wherein the preselected average particle size is lessthan 1,000 nanometers.
 7. The process of claim 1 wherein the preselectedaverage particle size is less than 600 nanometers.
 8. The process ofclaim 1 wherein the preselected average particle size is less than 300nanometers.
 9. A method for fabricating a thermoelectric material,comprising: providing an initial feedstock of silicon metalparticulates; providing an extracting liquid to extract oxidants fromthe silicon metal particulates; combining the silicon metal particulatesand the extracting liquid into a mixture and milling said mixture;withdrawing at least a portion of the milled mixture; within thewithdrawn portion of the milled mixture, separating milled silicon metalparticulates from the extracting liquid; and mixing the milled siliconmetal particulates with a dopant to form a thermoelectric material. 10.The method of claim 9 further comprising the step, following mixing themilled silicon metal particulates with a dopant, of sintering the milledsilicon metal particulates and dopant at a temperature below the meltingpoint of silicon.
 11. The method of claim 9 wherein the dopant is amaterial selected in order to make the thermoelectric material an n-typesemiconductor.
 12. The method of claim 9 wherein the dopant is amaterial selected in order to make the thermoelectric material a p-typesemiconductor.
 13. The method of claim 9 wherein the thermoelectricmaterial includes two sides, wherein a first side is an n-typesemiconductor and a second side is a p-type semiconductor.
 14. Athermoelectric material to exploit a unidirectional thermal gradient forthe production of electrical power, comprising: a body fabricated frommilled silicon alloyed with a dopant and sintered at a temperature belowthe melting point of silicon.
 15. The thermoelectric material of claim14 wherein the dopant is a material selected in order to make thethermoelectric material an n-type semiconductor.
 16. The thermoelectricmaterial of claim 14 wherein the dopant is a material selected in orderto make the thermoelectric material a p-type semiconductor.
 17. Thethermoelectric material of claim 14 wherein the body includes two sides,a first side being an n-type semiconductor and a second side being ap-type semiconductor.
 18. The thermoelectric material of claim 14wherein the milled silicon comprises particles with an average particlesize of less than 1,000 nanometers.
 19. The thermoelectric material ofclaim 14 wherein the milled silicon comprises particles with an averageparticle size of less than 600 nanometers.
 20. The thermoelectricmaterial of claim 14 wherein the milled silicon comprises particles withan average particle size of less than 300 nanometers.